I have started by 1940 Desoto for what Im guessing in over 10 years. The oil looked fresh and had good even compression readings. Oiled the cylinders and spun a few times before the first start. Car is not overheating but I do notice blowby through the filler neck white smoke from the exhaust.
What would be the best thing to begin testing or doing in order to eliminate this being that the car has sat for so long? Add a cleaner additive and change oil? add a ring sealer thereafter?
Some more info; I added a oil pressure guage and when cold it reads 40 psi once the car warms up it drops to 5-10 psi Is this huge decrease normal?
Any help is appreciated

Could be a stuck ring. I would run a compression check to see if all cylinders are within 10% of each other.
That oil pressure sounds a little low but I am not a flat head guy so dont go by me. I would at a minimum dump the oil and add fresh oil with some Marvel Mystery oil. This will take care of stuck rings a lot of the time. A fresh quality oil filter may help the pressure.

Good luck !!

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I have started by 1940 Desoto for what Im guessing in over 10 years. The oil looked fresh and had good even compression readings. Oiled the cylinders and spun a few times before the first start. Car is not overheating but I do notice blowby through the filler neck white smoke from the exhaust.
What would be the best thing to begin testing or doing in order to eliminate this being that the car has sat for so long? Add a cleaner additive and change oil? add a ring sealer thereafter?
Some more info; I added a oil pressure guage and when cold it reads 40 psi once the car warms up it drops to 5-10 psi Is this huge decrease normal?
Any help is appreciated

Marvel Mystery oil has a good track record for unsticking rings and lifters, you might give it a try. But if it has uniform compression readings, the problem might be elsewhere. As long as no water got into the cylinders, the engine should come back to the same condition it was before being parked. But if that condition was worn out, obviously nothing will change that.

Usually white smoke is water. Does the smoke have any odor to it? Could be a leaking head gasket, cracked head or block. Was the engine full of water or coolant? Does it freeze where the car was sitting? Is the coolant contaminated w/oil? ATF will also smoke white but has a distinctive odor. So does antifreeze. Water not so much.

I found the flathead six in my Rambler after a rebuild smoking more than I thought it should. I understand before it's broken in but damn this thing kept on smoking. I added "Restore" to it and it stopped after a little while, I don't know how long but all of a sudden it hit me that it wasn't smoking. I am not sure if it did anything or it simply would have fixed it's self with a little more wear or something. Restore USA - Engine Restorer FAQs

But you know what floors me, before I rebuilt the motor I ran it in the old parts car and it ran like a TOP, I mean perfect, idled like a watch, fired up in just a few cranks after 10-15 years in a bone yard, it really was a great runner. But it smoked like a mad dog and that is why I tore it down for rebuild. It had very little cyl wear and was in great shape, never really finding what was wrong to make it burn oil, just figured it was getting past the rings. I ran Marvell mystery oil in it for a while and it did nothing. Tried a number of tricks I heard about, did nothing. Well, I have to tell you, when I pulled out the fuel pump to have rebuilt I found that it was full of oil! It has a dual diaphragm pump so it pumps fuel and creates vacuum for the vacuum wipers. Well it is hooked up to a vacuum source on the head (the intake runners) and that goes to the pump then up from the pump is another hose to the wipers. I guess it uses the engine vacuum but then the pump keeps is constant so the wipers run smooth? (never did understand exactly what it was doing). So the pump had a seal that was bad and it was full of oil in the vacuum pump! There is a good chance that friggin pump was putting oil into the vacuum line going up to the head! That means my intake was SUCKING OIL into the cylinders and very likely was causing my smoking problem! I hate the thought of this after spending $2000 to rebuild the motor but I guess having it new is nice. But when it was smoking after getting it running I was pretty ticked at the thought that it may have been a good motor before the rebuild.

Anyway, you could give that Restore a try and look into your fuel pump to make a long story short.

I had a 351C pull ATF through the modulator. Was the strangest thing I've ever experienced:

The car was new to me, was slightly overfilled when I checked the oil. I did an immediate oil and filter change, tune up, rebuilt the brakes and put in A-arm bushings (a real chore), and started using it on a daily basis.

Checked the oil and after a week's time, the oil was a little high. I chalked it up to the car not being level when I changed the oil. Also noticed the ATF was a little low; topped it up.

Next time I checked the fluids the same dam thing. Let it go like that for a while longer until I was sure I was seeing the engine gain oil and the tranny lose fluid. Checked the modulator line and it was full of ATF.

But the strange thing was, there was NO visible smoke. Nothing. Nor did the tranny (FMX) shift abnormally in any way. A new modulator fixed the problem. Still puzzled as to how it could draw enough ATF to actually raise the motor oil level, but not smoke like a chimney. It was almost as though it was only doing it when the engine was off, but I know that couldn't be, unless there was some type of siphon action taking place...

This isn't going to help with the problem, I believe the '40 DeSoto was manual-only, but that may be wrong. But a fluid drive didn't use a modulator, either.

BTW, the '40 MOPARs used an electric wiper motor. But the AMC wiper/fuel pump deal is definitely a candidate for "Weird but True".

Thanks guys I drained the oil today, added some cheap 40w oil, drove it around the block a few times and the white smoke seemed to go away a bit. Is there a chance that once I flush the oil again and refill with some good quality oil and maybe an additive the smoke will slightly go away? Once again compression was really good and even according to flathead 6 specs.
I will try different things and see if it gets worse. It runs great. Newest thing I found out about this car is that It has a 3 speed tranny with an overdrive and according to the manual you cannot use SEA80 Gear oil because it has black oils and contaminants that can damage the overdrive. It calls for a SEA50 Cold tested engine oil.. or SEA50 aviation oil.
Would the SEA80 gear oils have the contaminants described in the 1940 shop manual?

Thanks guys I drained the oil today, added some cheap 40w oil, drove it around the block a few times and the white smoke seemed to go away a bit. Is there a chance that once I flush the oil again and refill with some good quality oil and maybe an additive the smoke will slightly go away? Once again compression was really good and even according to flathead 6 specs.
I will try different things and see if it gets worse. It runs great. Newest thing I found out about this car is that It has a 3 speed tranny with an overdrive and according to the manual you cannot use SEA80 Gear oil because it has black oils and contaminants that can damage the overdrive. It calls for a SEA50 Cold tested engine oil.. or SEA50 aviation oil.
Would the SEA80 gear oils have the contaminants described in the 1940 shop manual?

If the engine's running well, and seems to be getting better, I see no reason to not continue driving it.

Today's lubricants are a far cry from the crude (npi) lubes used in '40.

Eaton and CAT both use a 50w gear lube, but unless you learn something to the contrary I'd use a straight 50w motor oil.

You might want to ask over at P15 D24 forum, they cater to the old MOPARs.

Yeah, our 1940 Pontiac used a vacuum motor, my Dad converted it to electric. I can remember rain storms in FL where we'd end up having to stop because to make the wipers work fast required the vacuum to be high- so you'd let off the gas to try to see where you were going. Do that enough times and pretty soon you're going 20 mph in second gear just to see! lol

If the engine's running well, and seems to be getting better, I see no reason to not continue driving it.

Today's lubricants are a far cry from the crude (npi) lubes used in '40.

Eaton and CAT both use a 50w gear lube, but unless you learn something to the contrary I'd use a straight 50w motor oil.

You might want to ask over at P15 D24 forum, they cater to the old MOPARs.

I ended up using an SAE80 gear oil, which is what seems to had been in there before, Once I get better clarification from some Mopar experts I will swap over to the SAE50 motor oil if needed. Im not driving the car much just testing around block to test brakes, cooling, etc. Thank You

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